Driving wheels of an automobile are supported on a suspension system by a driving-wheel supporting hub unit. FIG. 8 shows the structure described in Patent Document 1, among such driving-wheel supporting units. This driving-wheel supporting hub unit rotatably supports a hub 2 and an inner ring 3 via a plurality of rolling elements 4 and 4 on the inner diameter side of an outer ring 1. Among these elements, the outer ring 1 does not rotate even during use, in a state where it is coupled with and fixed to a knuckle (not shown) constituting the suspension system. Moreover, first and second outer ring raceways 5 and 6 are provided at the inner circumferential surface of the outer ring 1 so as to rotatably support the hub 2 and the inner ring 3 on the internal diameter side of this outer ring 1 coaxially with the outer ring 1.
Hereinafter, with respect to an axial direction, “outer” indicates outside of a vehicle in its width direction in an assembled state to the vehicle. The “outer” indicates left side of FIG. 8 and lower side of FIGS. 1 to 5. On contrary, with respect to the axial direction, “inner” indicates the right side of FIG. 8 and the upper side of FIGS. 1 to 5 which become the central side of the vehicle in its width direction. These are all true in the present specification and the claims.
The hub 2 has a mounting flange 7 for supporting and fixing a vehicle wheel at a portion near an outer end of the outer circumferential surface, a first inner ring raceway 8 at an intermediate portion thereof, a small-diameter stepped portion 10 of which outer diameter becomes smaller than the portion where the first inner ring raceway 8 is formed at an inner end thereof and a spline hole 11 at a central portion. The inner ring 3 which has the second inner ring raceway 9 formed at its outer circumferential surface is fitted to the small-diameter stepped portion 10.
In addition, at the inner end of the inner circumferential surface of the inner ring 3, a large-diameter stepped portion 12 of which inner diameter becomes larger than the intermediate portion and the outer end is formed. Also, the inner end of the cylindrical portion 13 provided at the inner end of the hub 2 is plastically deformed to form a caulking portion 14 and a stepped surface 15 of the large-diameter stepped portion 12 is held by this caulking portion 14. As a result, the inner ring 3 is coupled with and fixed to the hub 2. The operation of plastically deforming the inner end of the cylindrical portion 13 radially outward as described above is performed by an orbital forging press. In this state, an inner end surface 16 of the inner ring 3 projects further axially inward than the caulking portion 14. Moreover, a plurality of rolling elements 4 and 4 are provided between the first and second outer ring raceways 5 and 6 and the first or second inner ring raceways 8 and 9, respectively.
When the driving-wheel supporting hub unit as mentioned above is assembled to an automobile, as shown in the drawing, a spline shaft 19 (driving shaft) provided in a central portion of an outer end surface 18 of an outer ring 17 for a constant-velocity joint is spline-engaged with the spline hole 11 of the hub 2. In addition, a radial outer end of the outer end surface 18 of the outer ring 17 for a constant-velocity joint is made to abut on the inner end surface 16 of the inner ring 3. In this state, a front end of the spline shaft 19 is screwed to a nut 20 and is further fastened. Thus, the hub 2 and the inner ring 3 are coupled with the spline shaft 19.
In such a conventional structure, the inner end surface of the caulking portion 14 is not made to abut on the outer end surface 18 of the outer ring 17 for a constant-velocity joint. For this reason, it is not necessary to finish the inner end surface of the caulking portion 14 into a flat surface perpendicular to a central axis. On the other hand, the inner end surface 16 of the inner ring 3 which is made to abut on the outer end surface 18 of the outer ring 17 for a constant-velocity joint is originally a flat surface perpendicular to the central axis. For this reason, it is not necessary to perform flattening again on the inner end surface 16 after the assembling of the hub unit. Accordingly, since such flattening can be omitted, the manufacturing cost of the driving-wheel supporting hub unit is reduced.
Meanwhile, in the aforementioned conventional technique, the caulking portion 14 is formed by an orbital forging press. Therefore, an apparatus to be used when the caulking portion 14 is formed becomes huge. This is because it is necessary to incorporate a mechanism which rocks the pressing die, a mechanism which prevents the hub unit from being swayed at the time of the operation of the orbital forging press, or the like into this apparatus. Anyway, if the apparatus becomes huge in this way, the manufacturing cost of the driving-wheel supporting hub unit increases.
On the other hand, a manufacturing method of forming the caulking portion 14 by hot forging is disclosed in Patent Document 2. Meanwhile, if the caulking portion 14 is formed by this hot forging in this way, the problems that, due to the heat generated during the hot forging, lubricating grease enclosed in a rolling-element setting portion deteriorates and thus the lifespan of the grease become short, or due to the heat, the inner end surface 16 of the inner ring 3 is distorted and it is thus difficult to make the contact pressure between the inner end surface 16 and the outer end surface 18 uniform over the whole periphery, may occur. Particularly when the working temperature of the caulking portion 14 exceeds a tempering temperature when the inner ring 3 is formed, a problem that the strength performance of this inner ring 3 degrades may occur. For this reason, the working temperature during cold forging as well as that of the hot forging becomes important.    Patent Document 1: Japanese Patent Unexamined Publication No. JP-A-9-164803    Patent Document 2: German Patent Unexamined Publication No. DE-A-3418440